This invention relates to a drive system employing two or more torque transmitting devices, and particular it relates to an improved clutch or brake arrangement for providing load sharing between two or more motors driving a common load.
Canadian Pat. No. 934,679 issued Oct. 2, 1973 to Eastcott et al, describes a clutch or brake inching scheme. This scheme has been found to be practical and has enjoyed acceptance in the mining industry for synchronous motor driven grinding mill drives where load sharing or angle matching between partner driving motors is essential. If the load is not shared equally between the driving motors, one motor would be overloaded while the other motor would be underloaded.
The scheme described in the aforementioned Canadian Pat. No. 934,679 pulses the clutches for slightly different durations, that is, the clutch connected with the motor having the greater load is pulsed for a slightly longer time than is the clutch connected with the motor having the lesser load. The term "pulse" or "pulsing" refers to a brief reduction of the fluid pressure in a pressure actuated clutch so that the pressure in the clutch drops briefly below the load line permitting the clutch to slip. This unequal pulsing tends to cause the overloaded motor to shed some of its load while the underloaded motor takes on more load.
It should be remembered that the rotational adjustments are very small. For example, a 6000 HP, 180 RPM synchronous motor operated from a 60 Hz supply will have a 100% load change over a load angle, measured at the clutch, of 1.4 degrees. This is only 84 minutes of arc. If acceptable load sharing between motors was determined to be plus or minus 4%, then this is equivalent to only 3.36 minutes of arc (or 0.056 degrees). This is a very small angular amount.
One of the difficulties of achieving good load sharing is that the pulse of air (that is the reduced pressure pulse) does not immediately drop from a stable operating clutch air pressure to the desired reduced air pressure which permits clutch slipping, nor does it immediately return to normal operating pressure at the end of the pulse. The present invention seeks to improve the form of the pulse.